The Administrator sets the position characteristics for each Position on each Match. The following image presents a single Match that demonstrates all three Methods. The Referee and Assessor position for Match #5 are going to be manually assigned by an Assignor, there will be no 4th Official, and the two AR positions are available for SelfAssign to properly qualified Referees.
Note that in some Chapters, the Assignors themselves may be authorized to set the Position Method. In other Chapters, the client Organizations themselves may be authorized to do so. Regardless of who actually sets the Method, the Site will respond accordingly.